1. Field of the Invention
This disclosure relates to a semiconductor device, and more particularly, to a boost voltage generating circuit and method of a semiconductor device.
2. Description of the Related Art
Semiconductor devices, particularly semiconductor memory devices, often use a boost voltage generating circuit for receiving a power supply voltage from an external source and generating a boost voltage VPP higher than the power supply voltage. Generally, the boost voltage generating circuit is referred to as a VPP pump.
FIG. 1 is a circuit diagram illustrating a conventional boost voltage generating circuit, and FIG. 2 is a circuit diagram illustrating a VPP level shifter shown in FIG. 1. FIG. 3 is a timing diagram illustrating the normal operation of the conventional boost voltage generating circuit shown in FIG. 1. And FIG. 4 is a timing diagram illustrating the power-up operation of the conventional boost voltage generating circuit shown in FIG. 1.
Referring to FIG. 1, the conventional boost voltage generating circuit includes unit pump circuits 11 and 13, an output control transistor N12, and an output controlling circuit 15. The conventional boost voltage generating circuit may include two or more unit pump circuits.
The unit pump circuit 11 boosts the voltage of a boost node BST0 in response to a control signal P1, and transfers charge from the boost node BST0 to a boost node BST1 of the unit pump circuit 13 through a transfer transistor NT0. The unit pump circuit 13 boosts the voltage of the boost node BST1 in response to a control signal P2, and transfers charge from the boost node BST1 to an output node VPP through a transfer transistor NT1.
The control signals P1 and P2 are non-overlapping pulse signals generated by a boost voltage detector (not shown) that detects the voltage of the output node VPP, and a signal VCCHB is a reset pulse signal generated during the power-up operation.
The conventional boost voltage generating circuit operates as follows. Referring to FIG. 4, if a power supply voltage VCC increases above the threshold voltage of an NMOS transistor during the power-up operation, the NMOS output control transistor N12 is turned on and thus the voltage of the output node VPP becomes VCC-VTN, obtained by subtracting the threshold voltage VTN of the NMOS transistor from the power supply voltage VCC. If the control signal P1 is toggled, the voltage of the output node VPP is held in the power supply voltage VCC until the signal VCCHB is setup. If the power supply voltage VCC increases during the power-up operation, the voltage of the output node VPP also increases.
If the voltage of the output node VPP increases above the voltage for operating the VPP level shifter 21 shown in FIG. 1, charge from the boost node BST1 is transferred to the output node VPP through the transfer transistor NT1, and the voltage of the output node VPP increases above the power supply voltage VCC. If the voltage of the output node VPP reaches a target value, the control signals P1 and P2 become low and thus the pumping operations of the pump circuits 11 and 13 are stopped. If the charge of the output node VPP is consumed by the operation of the semiconductor memory device, the pumping operations are performed again. The transistor and the inverter shown by dotted lines in FIG. 1 are the portions to which the VPP level is applied, and use thick oxide in order to ensure reliability.
FIG. 5 is a timing diagram illustrating the normal operation of the conventional boost voltage generating circuit shown in FIG. 1 at a low power supply voltage VCC. FIG. 6 is a timing diagram illustrating the power-up operation of the conventional boost voltage generating circuit at a low power supply voltage VCC as shown in FIG. 1.
If the power supply voltage VCC of the semiconductor memory device is decreased, the setup voltage of a power-up reset pulse signal VCCHB is also decreased. A transistor using thick oxide has a higher threshold voltage than a transistor using thin oxide. As shown in FIG. 6, if the setup voltage of the power-up reset pulse signal VCCHB is decreased below the threshold voltage of the PMOS transistor of the VPP level shifter 21, the boosting operation and the charge transfer operation are not performed well. Thus, the VPP level does not increase above the setup voltage of the power-up reset pulse signal VCCHB.
Also, although the setup voltage of the power-up reset pulse signal VCCHB increases above the threshold voltage of the PMOS transistor of the VPP level shifter 21 (FIG. 2), it takes much time to transfer the charge of the boost node BST1 to the output node VPP, due to the weak driving power of the transfer transistor NT1. The result is that an active cycle time of the semiconductor memory device is long. In some cases, insufficient charge is transferred from the boost node BST1 to the output node VPP, and thus the performance of the boost voltage generating circuit may severely deteriorate.